For example, Patent Literature 1 and the like recite techniques of attaching/detaching (mounting, dismounting) a boom to/from an upper slewing body. Patent Literature 1 recites, in FIG. 11, a technique of attaching/detaching a boom using a power device (“other lifting means” in Abstract of Patent Literature 1) separate from a crane to/from which the boom is attached/detached. Additionally, FIG. 1 to FIG. 8 and Abstract of Patent Literature 1 recite a technique of attaching/detaching a boom using a boom receiving base.